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3. Effects of tropical forest on water yield

Abstract
The tropical forest regions
Hydrological processes in tropical forests
Water yield characteristics
Methods of detecting the effects of forests on water yield
Effects of tropical forests on water yield
Conclusion
Appendix
Symbols and abbreviations
References
Assessment

L. Oyebande

Abstract

The tropical zone and the transition to the subtropical forests experience a range of humid climates. This together with the varied vegetation results in a variety of distinct hydrological regimes. The rapid exploitation of tropical forests makes it imperative that the effects on the hydrology of a region are understood. For this, the individual hydrological processes in the forest must be studied. The components of tropical forest evapotranspiration approach the meteorological potential value and this contribution to regional rainfall suggests that deforestation might reduce rainfall. The differences in organic matter decomposition on the forest floor are poorly understood and result in a variety of surface runoff conditions.

Many parameters of water yield from a catchment may be studied by stream flow measurements, but scientific criteria for calibration and treatment must be met. Generalizations on the effects of forest on water yield are drawn from all the successful catchment experiments in six tropical regions. In the first year after clearing forest, stream flow increased by a maximum of 6 mm for each of of the area cleared. The average was about 5 mm; variations are due to regrowth or different land uses. More experiments are needed in tropical areas to elucidate the causes of the observed effects of forest clearance.

The tropical forest regions

According to Tricart (1965), the forested tropics corresponding to the evergreen and semi-evergreen seasonal forests are part of the humid tropics where the most extreme morphogenic conditions are realized through the combination of high temperatures and humidities. WMO (1983) defined the humid tropical regions as being "located principally between the Tropics of Cancer and Capricorn and having a mean annual precipitation of at least 1,000 mm and a mean monthly temperature in any month of at least 20°C." These regions coincide more or less with the areas defined by Köppen in his class Af (see Appendix) as tropical rainy climates (fig. 1). Three biochores share the humid tropics: forest, bush, and savanna. Forest occupies regions without too marked a seasonal drought. In West Africa the minimum range of annual rainfall required to support forest is 1,200 -1,500 mm depending on the nature of the regolith. But in Latin America, numerous forests grow with annual rainfalls of only 1,000-1,100 mm.

The problem of delimiting the humid tropical regions where the natural or climax vegetation is the tropical rain forest is a delicate one. In Atlantic Brazil, Vietnam, and the south of China, for instance, temperatures drop (to 15°C in Hong Kong) while the forest continues uninterrupted. Furthermore, although the forest gradually changes, it extends beyond the tropics. The subtropical and montane rain forests of these regions, and those of South Chile and South Africa together with the more seasonal types of evergreen forests found in areas with a somewhat marked dry season, are included in the region of tropical forest for the purpose of the present report. The tropical forest occurs in three regions: the American, African, and Indo-Malayan regions. In all three regions the forest is normally restricted to lowlands and to slopes below an altitude of 1,500 m (fig. 1). The American forests include the Amazon Basin, which still contains the world's largest tropical rain forest and still carries a sparse population. The forest of Amazonia covers over 6.6x 106 km² and is shared by Brazil, Bolivia, Ecuador, Peru, Colombia, and Venezuela. The American region extends northwards into Mexico and into the chain of islands in the Caribbean, especially Cuba and the Dominican Republic, Puerto Rico, and Haiti.

The African region of tropical forest is centred on the Congo Basin, from where it extends westwards along the Guinea Coast of West Africa. Forest zone occurs in a 150300 km wide belt along most of the coast from Gabon and Cameroon westwards to Sierra Leone, except for a gap from eastern Ghana to the Benin Republic, where there is a breach of dry climate reaching to the coast. Southwards and eastwards, the forest extends towards Zimbabwe. Some of it is found in Seychelles, Mauritius, and on the east coast of Madagascar. Much of the land in the African forest region is occupied by farmland either under cultivation or as bush fallow. Most of the forest is secondary, and untouched or primary forest is rare except in forest reserves.

The Indo-Malayan region extends from western India and Sri Lanka to Thailand, IndoChina, and the Philippines, as well as through Malaysia to New Guinea and the adjoining islands of Sumatra and Borneo. In India, it is also found in the northwestern and eastern Ghats and more extensively in the lower parts of the eastern Himalayas, the Khasi Hills, and Assam (Richards 1964). The Indo-Malayan region continues southward into Australia and the islands of the Pacific. The southern boundary is approximately 17°18°S and lies well north of the Tropic of Capricorn. However, the forest continues south to the Tropic of Capricorn in a narrow strip along the east coast of Australia.

FIG. 1. World distribution of tropical rain forest (after Trewartha 1961).

TABLE 1. Conversion of tropical forests in Central America

Source: Myers 1981

Present Area of Tropical Forests

Increased cutting since 1950 is believed to have led to the loss of half of the world's tropical forests. It is estimated that at the current rate over two-thirds of the remaining forest will have been lost by the year 2000.

Estimates of the total remaining tropical forests amount to about 9 x 10⁶ km², with 5.4 x 10⁶ km² in the American region, 1.9 x 10⁶ km² in the African region, and 2.7 x 10⁶ km² in the Indo-Malayan region of the Asia-Pacific region (Brunig 1977). About 4 x 10⁶ km² of this area is under shifting cultivation. In regional terms the pattern of deforestation is as follows: 10,000 km² of South and Central American forests are being cleared each year. In Central America only about one-third of the original forest remains and in Costa Rica alone 550 km² are being lost annually to cattle production for North America (table 1). Myers (1981) refers to this phenomenon as the "Hamburger Connection." On the other hand in French Guiana, part of the Amazon Basin, forest reserves cover some estimated 98,000 km² or 98% of the country and, like the rest of the basin, it still has a sparse population.

In South-East Asia (Thailand, Malaysia, Indonesia, Vietnam, Laos, and Kampuchea) more than 150,000 km² of forests are cleared each year. The example of the Indonesian province of East Kalimantan illustrates the impact of man in the Indo-Malayan region. The large province is approximately 211,000 km², or 10% of the total area of Indonesia, with a relatively small population. 173,000 km² of the province is forested with an estimated timber volume of 800x 10⁶ m³. Of this area some 130,000 km² is allocated to commercial mechanized logging. The extensive timber operations over the years have considerably damaged the remaining forest (Kartawinata et al. 1981) and caused soil compaction and other damage Shifting cultivation has also been practised for a long time and has, in places, resulted in the formation of "derived" grasslands (4,000 km² at present) and secondary forests. The latter is of the order of 24,000 km² in extent.

In Africa, more than 1 x 106 km² of the tropical forests have been removed and every year large areas are being converted to "derived" savannas and secondary forests. In general most of the tropical forest regions, including the Amazon, are undergoing intensive programmes of development, and it is important that some appraisal be made of the effects of such change. As it happens, our present knowledge of the dynamics of the forest cover and its interactions with the hydrological system in these regions is still very modest, lagging far behind that of other climatic zones. The objective of this report is to examine critically the available evidence as to the effect of these forests on water yield with a view to appraising the potential effects of changes in the forest cover.

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